The trend towards designing and building fuel efficient, low or zero emission on-road and off-road vehicles has increased dramatically in recent years, with significant emphasis being placed on the development of hybrid and all-electric vehicles. This has led, in turn, to a greater emphasis being placed on electric motors, either as the sole source of propulsion (e.g., all-electric vehicles) or as a secondary source of propulsion in a combined propulsion system (e.g., hybrid or dual electric motor vehicles). The electric motor in such an application may utilize either an AC or DC permanent magnet motor design or an AC induction motor design. Regardless of the type of electric motor, it must be designed to achieve the desired efficiency, torque density and high speed torque with an acceptable motor size and weight.
In a multi-phase AC induction motor, a rotating magnetic field is generated by a plurality of circumferentially distributed multi-phase coil windings secured within a plurality of circumferentially distributed slots in the inner periphery of the motor's stator, the coil windings being coupled to a multi-phase AC power source controlled with certain desired frequencies and certain desired levels of voltage or current in each phase. The magnetic field generated within the stator core induces multiple-phase alternating currents in the rotor windings which in turn interact with the stator magnetic field. The resultant rotating field causes the desired shaft torque and rotation of the motor's rotor at the desired speed, the rotor being comprised of one or more magnetic pole pairs with the same number of pole-pairs as that of the stator windings of each phase.
For decades AC induction motors have been the work-horses of modern society, such motors being designed and manufactured with a variety of characteristics to match the vast range of desired applications. In general, the various electrical, thermal and mechanical aspects of a motor are designed to meet the performance specifications and cost constraints for a specific application. On one end of the spectrum of applications economic considerations dominate, as exhibited by manufacturing and maintenance costs, such applications including appliances, factory process controls, and most other applications of induction motors. On the other end of the spectrum of applications performance dominates, where high performance requirements such as high power density and high dynamic response are met using specific materials and manufacturing processes, typically at higher costs. Some applications, however, require both high performance and low cost. For example, electric vehicles have very demanding performance requirements, e.g., high efficiency, high torque density, high power factor and drive converter utilization, wide constant power range at high speeds, high speed torque capability, high maximum speed, while also requiring that the resultant motors achieve high reliability, small size, low weight, mass manufacturability and low cost.
Accordingly, what is needed is a low cost and easily manufactured electric motor which achieves the very demanding performance requirements of electric and hybrid vehicles. The present invention provides such a motor design.